http://www.google.com/search?hl=en&q=parser

:) No seriously, check out ANTLR.  Unless you are supposed to write the
parser from scratch.
If you want to do it in ruby, there are options like:
http://split-s.blogspot.com/2005/12/antlr-for-ruby.html
http://www.zenspider.com/ZSS/Products/CocoR/